This invention relates to liquid hydrocarbon fuels having improved ignition characteristics, more particularly to diesel fuels with enhanced cetane numbers. 2. Description of the Art
Fuel ignition in diesel engines is achieved through the heat generated by air compression, as a piston in a cylinder moves to reduce the cylinder volume during a compression stroke. In the engine, the air is first compressed, then fuel is injected into the cylinder; as fuel contacts the heated air, it vaporizes and finally begins to burn as the self-ignition temperature is reached. Additional fuel is injected during the compression stroke and this fuel burns almost instantaneously, once the initial flame has been established.
Thus, a period of time elapse between the beginning of fuel injection and the appearance of a flame in the cylinder. This period is commonly called "ignition delay" and must be quite short to avoid "diesel knock," which is caused by the accumulation and detonation of relatively large amounts of fuel before the desired smooth combustion process has been established.
A reduction in ignition delay can be obtained by varying the chemical nature of the injected fuel. Straight-chain paraffinic hydrocarbons give the least ignition delay, while branched-chain paraffins and cyclic (including aromatic) compounds tend to have poorer ignition characteristics. For this reason, n-hexadecane ("cetane"), which is diesel fuel having excellent ignition qualities, has long been used as a standard reference material for determining the ignition quality of commercial diesel fuels. A scale called "cetane number" has been devised for ranking the relative ignition delay characteristics of a given diesel fuel. The cetane number of an unknown fuel is determined by comparing its ignition delay in a standard test engine with reference fuels which are prepared by blending cetane (assigned a rating of 100) and 2,2,4,4,6,8,8,-heptamethyl nonane (assigned a rating of 15) until a reference fuel is found to have the same ignition delay characteristics as the unknown fuel; the cetane number is obtained by the equation: EQU Cetane No.=volume % cetane+0.15 (volume % heptamethylnonane)
In general, large stationary engines which run at fairly constant speeds and loads have the lowest cetane number requirements (e.g., 30 to 45), while smaller, motor vehicle diesel engines have the highest requirements (e.g., 40 to 55) for obtaining optimum performance. In addition to decreasing ignition delay, a higher cetane number fuel facilitates low temperature starting, provides smoother engine operation, and decreases engine deposits.
A modern petroleum refinery can produce high quality diesel fuels with large straight-chain paraffin contents. However, due to competing demands for other products, limitations imposed by poor quality heavy crude oils, and other factors, a refinery frequently is unable to meet the total demand for such diesel fuels. Because of these refining constraints, various additives have been used to increase the cetane number of diesel fuels, thereby permitting a refiner to produce larger volumes of fuel which, without additives, would not be acceptable to some consumers.
Moran et al., in U.S. Pat. No. 2,115,275, describe diesel fuels which have a reduced tendency to exhibit knocking, prepared by mixing a trizo compound with the fuel.
U.S. Pat. No. 2,188,262 to Crandall is directed to diesel fuel compositions which have improved ignition characteristics, due to the presence of a pentazdiene additive.
Miller et al. in U.S. Pat. No. 2,225,879, teach that cetane number increases are obtained by adding to diesel fuel organic compounds having a diazo group and a carboxy group in the same molecule.
U.S. Pat. No. 4,280,819 to Hartle et al. discloses an improvement in the cetane number rating of diesel fuel by adding thereto certain aromatic azide compounds.
Frankel et al., in their U.S. Pat. No. 4,303,414, describe additives which increase the octane rating of gasoline, enhance the centane rating of diesel fuel, and provide other benefits to internal combustion engines. The additives are alkyl azides.
A middle distillate fuel additive is taught by Irish in U.S. Pat. No. 4,365,973. One component of the additive is a paraffinic nitrate or mixture of nitrates, included for the purpose of cetane improvement.